Variable geometry turbine

ABSTRACT

A variable geometry turbine has an annular inlet passageway defined between a radial wall of a moveable wall member and a facing wall of the turbine housing. The moveable wall member is mounted within an annular cavity provided within the housing and having inner and outer annular surfaces. An annular seal is disposed between an annular flange of the moveable wall member and the adjacent inner or outer annular surface of the cavity. One or more inlet bypass passages are provided in the annular flange or said adjacent cavity surface, such that the annular seal and bypass passageways move axially relative to one another as the moveable wall member moves. The annular seal and the or each bypass passage are axially located such as the annular wall member approaches the facing wall of the housing the or each bypass passage permits the flow of exhaust gas through said cavity to the turbine wheel thereby bypassing the annular inlet passageway.

[0001] The present invention relates to a variable geometry turbine.Particularly, but not exclusively, the invention relates to the turbineof a turbocharger for an internal combustion engine. More particularlystill, the invention relates to vehicle engine turbochargers which maybe controlled to operate as an engine exhaust brake.

[0002] Turbochargers are well known devices for supplying air to theintake of an internal combustion engine at pressures above atmospheric(boost pressures). A conventional turbocharger essentially comprises anexhaust gas driven turbine wheel mounted on a rotatable shaft within aturbine housing. Rotation of the turbine wheel rotates a compressorwheel mounted on the other end of the shaft within a compressor housing.The compressor wheel delivers compressed air to the engine intakemanifold. The turbocharger shaft is conventionally supported by journaland thrust bearings, including appropriate lubricating systems, locatedwithin a central bearing housing connected between the turbine andcompressor wheel housings.

[0003] In known turbochargers, the turbine stage comprises a turbinechamber within which the turbine wheel is mounted, an annular inletpassageway defined between facing radial walls arranged around theturbine chamber, an inlet arranged around the inlet passageway, and anoutlet passageway extending from the turbine chamber. The passagewaysand chambers communicate such that pressurised exhaust gas admitted tothe inlet chamber flows through the inlet passageway to the outletpassageway via the turbine and rotates the turbine wheel. It is alsowell known to trim turbine performance by providing vanes, referred toas nozzle vanes, in the inlet passageway so as to deflect gas flowingthrough the inlet passageway towards the direction of rotation of theturbine wheel.

[0004] Turbines may be of a fixed or variable geometry type. Variablegeometry turbines differ from fixed geometry turbines in that the sizeof the inlet passageway can be varied to optimise gas flow velocitiesover a range of mass flow rates so that the power output of the turbinecan be varied to suite varying engine demands. For instance, when thevolume of exhaust gas being delivered to the turbine is relatively low,the velocity of the gas reaching the turbine wheel is maintained at alevel which ensures efficient turbine operation by reducing the size ofthe annular inlet passageway.

[0005] In one known type of variable geometry turbine, one wall of theinlet passageway is defined by an axially moveable wall member,generally referred to as a nozzle ring. The position of the nozzle ringrelative to a facing wall of the inlet passageway is adjustable tocontrol the axial width of the inlet passageway. Thus, for example, asgas flowing through the turbine decreases the inlet passageway width mayalso be decreased to maintain gas velocity and optimise turbine output.Such nozzle rings essentially comprise a radially extending wall andinner and outer axially extending annular flanges. The annular flangesextend into an annular cavity defined in the turbine housing (a part ofthe housing which in practice be provided by the bearing housing) whichaccommodates axial movement of the nozzle ring.

[0006] The nozzle ring may be provided with vanes which extend into theinlet passageway and through slots provided on the facing wall of theinlet passageway to accommodate movement of the nozzle ring.Alternatively, vanes may extend from the fixed wall through slotsprovided in the nozzle ring. Generally the nozzle ring is supported onrods extending parallel to the axis of rotation of the turbine wheel andis moved by an actuator which axially displaces the rods. Various formsof actuator are known for use in variable geometry turbines, includingpneumatic, hydraulic and electric actuators, mounted externally of theturbocharger and connected to the variable geometry system viaappropriate linkages.

[0007] In addition to the conventional control of a variable geometryturbine to optimise turbocharger performance, it is also known to takeadvantage of the facility to minimise the turbocharger inlet to providean exhaust braking function. Exhaust brake systems of various forms arewidely fitted to vehicle engine systems, in particular to compressionignition engines (diesel engines) used to power large vehicles such astrucks. Conventional exhaust brake systems comprise a valve in theexhaust line from the engine which when activated substantially blocksthe engine exhaust (fully blocking he exhaust line would stall theengine). This creates back pressure which retards rotation of the engineproviding a braking force which is transmitted to the vehicle wheelsthrough the vehicle drive train. The exhaust braking may be employed toenhance the effect of the conventional friction brakes acting on thevehicle wheels, or in some circumstances be used independently of thenormal wheel braking system, for instance to control down hill speed ofa vehicle. With some exhaust brake systems the brake is set to activateautomatically when the engine throttle is closed (i.e. when the driverlifts his foot from the throttle pedal), and in others the exhaust brakemay require manual activation by the driver, such as depression of aseparate brake pedal. The exhaust brake valve is generally controllableto modulate the braking effect, for example to maintain a constantvehicle speed.

[0008] With a variable geometry turbine it is not necessary to provide aseparate exhaust brake valve. Rather, the turbine inlet passageway maysimply be closed to its minimum flow area when braking is required. Thelevel of braking may be modulated by control of the inlet passagewaysize by appropriate control of the axial position of the nozzle ring (orother variable geometry mechanism). Whilst having the advantage ofobviating the need to provide a separate exhaust brake valve, there arehowever problems associated with operation of variable geometry turbinesin an exhaust braking mode.

[0009] In particular with the modem highly efficient turbines, arelatively high air flow is still delivered to the engine as the inletpassageway is reduced towards the minimum width. This can result inengine cylinder pressures approaching or exceeding acceptable limits ifthe inlet passage is closed too far. Accordingly there is a practicallimit on the extent to which the inlet passage can be closed in brakingmode, which in turn limits the effective braking force that can beprovided by control of a conventional variable geometry turbine.

[0010] It is an object of the present invention to obviate or mitigatethe above disadvantage.

[0011] According to the present invention there is provided a variablegeometry turbine comprising a turbine wheel supported in a housing forrotation about a turbine axis, an annular inlet passage way extendingradially inwards towards the turbine wheel, the annular inlet passagewaybeing defined between a radial wall of a moveable wall member and afacing wall of the housing, the moveable wall member being mountedwithin an annular cavity provided within the housing and having innerand outer annular surfaces, the wall member being moveable axiallybetween first and second positions to vary the width of the inletpassage way, the second axial position being closer to the said facingwall of the housing than the first axial position, the moveable wallmember having a first annular flange extending axially from the radialwall into said cavity in a direction away from said facing wall of thehousing, a first annular seal being disposed between said first annularflange and the adjacent inner or outer annular surface of the cavity,said first annular seal being mounted to one of said first annularflange or said adjacent annular surface of the cavity;

[0012] wherein one or more inlet bypass passages are provided in theother of said first annular flange and said adjacent cavity surface,such that said first annular seal and bypass passageways move axiallyrelative to one another as the moveable wall member moves between saidfirst and second positions; and

[0013] wherein said first annular seal and the or each bypass passageare axially located such that with the annular wall member in said firstposition the seal prevents exhaust gas flow through the cavity but withsaid moveable wall member in the second position the or each bypasspassage permits the flow of exhaust gas through said cavity to theturbine wheel thereby bypassing the annular inlet passageway.

[0014] Embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

[0015]FIG. 1 is a cross-sectional illustration of a prior artturbocharger;

[0016]FIGS. 2a and 2 b illustrate a modification of the turbocharger ofFIG. 1 in accordance with the present invention;

[0017]FIGS. 3a and 3 b illustrate a second embodiment of the presentinvention; and

[0018]FIGS. 4a and 4 b schematically illustrate a third embodiment ofthe present invention.

[0019] Referring to FIG. 1, this illustrates a known turbocharger asdisclosed in U.S. Pat. No. 5,044,880. The turbocharger comprises aturbine stage 1 and a compressor stage 2. The turbine stage 1 is avariable geometry turbine comprising a turbine housing 3 defining avolute or inlet chamber 4 to which exhaust gas from an internalcombustion engine (not shown) is delivered. The exhaust gas flows fromthe inlet chamber 4 to an outlet passageway 5 via an annular inletpassageway 6 defined on one side by a radial wall 7 of a moveableannular member 8, referred to herein as a nozzle ring, and on the otherside by a facing radial wall 9 of the housing 3. An array of nozzlevanes 10 extend through slots in the nozzle ring 8 across the inletpassageway 6 from a vane support ring 11 which is mounted on supportpins 12. The arrangement is such that the degree to which the vanes 10extend across the inlet passageway 6 is controllable independently ofthe nozzle ring 8 and will not be described in detail here.

[0020] Gas flowing from the inlet chamber 4 to the outlet passageway 5passes over a turbine wheel 12 which as a result drives a compressorwheel 13 via turbocharger shaft 14 which rotates on bearing assemblies15 located within a bearing housing 16 which connects the turbinehousing 2 to a compressor housing 17. Rotation of the compressor wheel13 draws in air through a compressor inlet 18, and delivers compressedair to the intake of the engine (not shown) via an outlet volute 19. Itwill be appreciated that the bearing housing also houses oil supply andseal arrangements, the details of which are not necessary for anunderstanding of the present invention.

[0021] The nozzle ring 8 comprises a radially extending annular portiondefining the radial wall 7, and axially extending inner and outerannular flanges 20 and 21 respectively which extend into an annularcavity 22 provided in the turbine housing 3. With the turbineconstruction shown in the figures, the majority of the cavity 22 is infact defined by the bearing housing 16—this is purely a result of theconstruction of the particular turbocharger to which the invention is inthis instance is applied and for the purposes of the present inventionno distinction is made between the turbine housing and bearing housingin this regard.—The cavity 22 has a radially extending annular opening23 defined between radially inner and outer annular surfaces 24 and 25.A seal ring 26 is located in an annular groove provided in outer annularsurface 25 and bears against the outer annular flange 21 of the nozzlering 8 to prevent exhaust gas flowing through the turbine via the cavity22 rather than the inlet passageway 6.

[0022] A pneumatically operated actuator 27 is operable to control theposition of the nozzle ring 8 via an actuator output shaft 28 which islinked to a stirrup member 29 which in turn engages axially extendingguide rods 30 (only one of which is visible in the figures) whichsupport the nozzle ring 8 via linking plates 31. Accordingly, byappropriate control of the actuator 27 the axial position of the guiderods and thus of the nozzle ring 8 can be controlled. FIG. 1 shows thenozzle ring 8 in its fully open position in which the inlet passageway 6is at its maximum width.

[0023] As mentioned above, a variable geometry turbine such as thatdisclosed in FIG. 1 can be operated to function as an exhaust brake byclosing the inlet passageway 6 to a minimum width when braking force isrequired. However, also as mentioned above, with such an arrangement theminimum width of the inlet passageway under exhaust braking conditionsis limited by the need to avoid unacceptably high engine cylinderpressures.

[0024]FIGS. 2a and 2 b illustrate a modification of the turbocharger ofFIG. 1 in accordance with the present invention. Only those parts of theturbine which need to be described for an understanding of the inventionare shown in FIGS. 2a and 2 b which are enlargements of the nozzlering/inlet passageway region of the turbocharger showing the nozzle ringin fully open and fully closed positions respectively. The nozzle ring 8is modified by the provision of a circumferential array of apertures 32provided through the radially outer flange 21. The positioning of theapertures 32 is such that they lie on the side of the seal ring 26remote from the inlet passageway 6 (as shown in FIG. 2a) except when thenozzle ring 6 approaches the closed position, at which point theapertures 32 pass the seal 26 (as shown in FIG. 2b). This opens bypassflow path allowing some exhaust gas to flow from the inlet chamber 4 tothe turbine wheel 12 via the cavity 22 rather than through the inletpassageway 6. The exhaust gas flow that bypasses the inlet passageway 6,and nozzle vanes 10, will do less work than the exhaust gas flow throughthe inlet passageway 6 particularly since this is turned in a tangentialdirection by the vanes 10. In other words, as soon as the apertures 32are brought into communication with the inlet passageway 6 there is animmediate reduction in the efficiency of the turbocharger andcorresponding drop in compressor outflow pressure (boost pressure) withan accompanying drop in engine cylinder pressure.

[0025] Thus, with the present invention the provision of the inletbypass apertures 32 will have no effect on the efficiency of theturbocharger under normal operating conditions but when the turbine isoperated in an engine braking mode, and the inlet passageway is reducedto its minimum, the apertures will 32 facilitate a greater reduction ininlet passageway size than is possible with the prior art without overpressurising the engine cylinders. This thereby provides improved enginebraking performance.

[0026] It will be appreciated that the efficiency reducing effect on theturbocharger can be predetermined by appropriate selection of thenumber, size, shape and position of the apertures 32.

[0027]FIGS. 3a and 3 b illustrate a second embodiment of the presentinvention. As with FIGS. 2a and 2 b, only detail of the nozzlering/inlet passageway region of the turbine is illustrated. Whereappropriate, the same reference numerals are used in FIGS. 3a and 3 b asused in FIGS. 1 and 2. FIGS. 3a and 3 b illustrate application of theinvention to an otherwise conventional turbine which differs from theturbine of FIG. 1 in several respects. Firstly, the nozzle vanes 10 aremounted on the nozzle ring 8 and extend across the inlet passageway 6and into a cavity 33 via respective slots provided in a shroud plate 34which together with the radial wall 7 of the nozzle ring 8 defines thewidth of the inlet passageway 6. This is a well known arrangement.

[0028] Secondly, in accordance with the teaching of European patentnumber 0 654 587, pressure balancing apertures 35 are provided throughthe radial wall 7 of the nozzle ring 8 and the inner annular flange 20is sealed with respect to the housing 3 by a respective seal ring 36located in an annular groove provided in the radially inner annularportion 24 of the housing 3. The provision of the apertures 35 ensuresthat pressure within the cavity 22 is equal to the static pressureapplied to the radial face 7 of the nozzle ring 8 by exhaust gas flowthrough the inlet passageway 6. This reduces the load on the nozzle ringwith an increase in the accuracy of control of the position of thenozzle ring 8, particularly as the inlet passageway 6 is reduced towardsits minimum width.

[0029] In view of the provision of a radially inner seal ring 36,application of the present invention requires provision of gas bypasspassages 32 a in the inner annular flange 20 of the nozzle ring 8. Thepassages 32 a are positioned relative to the seal ring 26 so that theyopen into communication with the inlet passageway side of the seal ring26 at the same time as passages 32 b in outer annular flange 21 therebyproviding a bypass flow passage through the cavity 22 achieving exactlythe same effect as described above in relation to the embodiment ofFIGS. 2a and 2 b.

[0030] Alternatively the outer passages 32 b can be omitted, relying onthe pressure balancing apertures 35 to provide a bypass flow path inconjunction with inner passages 32 a.

[0031] It is also known to seal the nozzle ring with respect to thehousing by locating inner and/or outer seal rings within locatinggrooves provided on the nozzle ring rather than locating groovesprovided within the housing. In this case the seal ring(s) will movewith the nozzle ring. Specifically, FIGS. 4a and 4 b illustrate thenozzle ring/inlet passageway region of the turbine disclosed in Europeanpatent number 0 654 587 (mentioned above) modified in accordance withthe present invention. Where appropriate, the same reference numeralsare used in FIGS. 4a and 4 b as are used above. As with the turbinearrangement of FIGS. 3a and 3 b, the nozzle vanes 10 are supported bythe nozzle ring 8 and extend across the inlet passageway 6, through ashroud plate 34 and into a cavity 33. Pressure balancing apertures 35are provided through the radial wall 7 of the nozzle ring 8, which issealed with respect to the cavity 22 by inner and outer ring seals 26and 37. However, whereas the seal ring 26 is located within a grooveprovided in the housing 3, the radially outer seal ring 37 is locatedwithin a groove 38 provided within the outer annular flange 21 of thenozzle ring 8 and thus moves as the nozzle ring moves.

[0032] In accordance with the present invention the inner annular flange20 of the nozzle ring 8 is provided with inlet bypass apertures 32 whichpass the seal ring 26 as the nozzle ring moves to close the inletpassageway 6 to a minimum (as illustrated in FIG. 4b). However, theouter inlet bypass path is provided not by apertures through the nozzlering, but by a circumferential array of recesses 39 formed in the outerannular portion 25 of the opening 23 of cavity 22. As can be seen fromFIG. 4a, under normal operating conditions the seal ring 37 will bedisposed inward of the recesses 39 preventing the passage of exhaust gasaround the nozzle ring 8 and through the cavity 22. However, as thenozzle ring moves to close the inlet passageway 6 to a minimum, as shownin FIG. 4b, the seal ring 37 moves into axial alignment with therecesses 39 which thereby provide a bypass path around the seal ring 37to allow gas to flow through the cavity 22, and to the turbine wheel viathe inlet bypass apertures 32 provided in the inner annular flange ofthe nozzle ring 8. It will be appreciated that the effect of therecesses 39 is directly equivalent to the effect of apertures 32 andthat in operation this embodiment of the invention will function insubstantially the same way as the other embodiments of the inventiondescribed above.

[0033] It will be appreciated that modifications may be made to theembodiments of the invention described above. For instance, if only oneseal ring is required as for example in embodiment of FIG. 8, and thisis located on the nozzle ring, then there will be no need to provideaperture 32 in the inner flange of the nozzle ring. Similarly, if thereare both inner and outer seal rings located in the housing, it will benecessary to provide bypass recesses in both the inner and outer annularportions of the housing instead of bypass apertures through the nozzlering.

[0034] Other possible modifications and applications of the presentinvention will be readily apparent to the appropriately skilled person.

1. A variable geometry turbine comprising a turbine wheel supported in ahousing for rotation about a turbine axis, an annular inlet passagewayextending radially inwards towards the turbine wheel, the annular inletpassageway being defined between a radial wall of a moveable wall memberand a facing wall of the housing, the moveable wall member being mountedwithin an annular cavity provided within the housing and having innerand outer annular surfaces, the wall member being moveable axiallybetween first and second positions to vary the width of the inletpassage way, the second axial position being closer to the said facingwall of the housing than the first axial position, the moveable wallmember having a first annular flange extending axially from the radialwall into said cavity in a direction away from said facing wall of thehousing, a first annular seal being disposed between said first annularflange and the adjacent inner or outer annular surface of the cavity,said first annular seal being mounted to one of said first annularflange or said adjacent annular surface of the cavity; wherein one ormore inlet bypass passages are provided in the other of said firstannular flange and said adjacent annular cavity surface, such that saidfirst annular seal and bypass passageways move axially relative to oneanother as the moveable wall member moves between said first and secondpositions; and wherein said first annular seal and the or each bypasspassage are axially located such that with the annular wall member insaid first position the seal prevents exhaust gas flow through thecavity but with said moveable wall member in the second position the oreach bypass passage permits the flow of exhaust gas through said cavityto the turbine wheel thereby bypassing the annular inlet passageway. 2.A variable geometry turbine according to claim 1, wherein the annularseal is mounted to said annular surface of the cavity, and the or eachbypass passage comprises an aperture through said first annular flangeof the moveable wall member.
 3. A variable geometry turbine according toclaim 2, wherein the annular seal is located within an annular grooveprovided within said annular surface of the cavity.
 4. A variablegeometry turbine according to claim 1, wherein said annular seal ismounted to said first annular flange, and the or each bypass passagecomprises a recess provided in the adjacent annular surface of thecavity.
 5. A variable geometry turbine according to claim 4, whereinsaid annular seal is located within an annular groove provided in anouter surface of said first annular flange.
 6. A variable geometryturbine according to claim 1, wherein said first annular flange extendsaxially from the radially outermost periphery of the radial wall of themoveable wall member, and said first annular seal is disposed betweenthe first annular flange and said outer surface of the cavity.
 7. Avariable geometry turbine according to claim 6, wherein said moveablewall member comprises a second annular flange extending axially intosaid cavity from the radially innermost periphery of the radial wall,and a second annular seal is mounted to one of said second annularflange or said inner annular surface of the cavity, and one or moreinlet bypass passages are provided in the other of said second annularflange and said inner cavity surface.
 8. A variable geometry turbineaccording to claim 1, wherein said first annular flange extends axiallyinto said cavity from the radially innermost periphery of the radialwall, and said first annular seal is disposed between the first annularflange and said inner surface of the cavity.
 9. A variable geometryturbine according to claim 8, wherein apertures are provided throughsaid radial wall of the moveable wall member whereby the cavity is influid communication with the inlet passageway, said apertures beingarranged such that in use a resultant force is exerted on the moveablewall member which force is always in a single axial direction.
 10. Avariable geometry turbine according to claim 1, wherein when themoveable wall member is in said second position the width of the inletpassageway is at a minimum.
 11. A variable geometry turbine according toclaim 1, wherein when the moveable wall member is between said firstposition and a third position intermediate the first and secondpositions the or each seal prevents flow of gas through said cavity viasaid bypass passages, and wherein when said moveable wall member isbetween said third position and said second position said bypasspassages permit the flow of gas through said cavity, and wherein saidthird position is closer to said second position than to said firstposition.
 12. A variable geometry turbine according to claim 11, whereinthe positions between said first and third positions correspond to anormal high efficiency operating mode of the turbine, and positionsbetween said third and second position correspond to an exhaust brakingmode of operation of the turbine.
 13. A variable geometry turbineaccording to claim 1, comprising a plurality of said bypass passagescircumferentially arranged around the respective annular flange orannular cavity surface.
 14. A variable geometry turbine according toclaim 1, comprising nozzle vanes extending into the inlet passage way.15. A variable geometry turbine according to claim 1, wherein saidnozzle vanes extend from said nozzle ring.
 16. A turbocharger comprisinga variable geometry turbine according to claim
 1. 17. A turbochargedinternal combustion engine comprising a turbocharger according to claim15.
 18. A powered vehicle comprising a turbocharged internal combustionengine according to claim 17, wherein operating said turbocharger withthe annular wall member at or adjacent said second position provides anexhaust braking function.